Portable communication receivers, such as selective call receivers, receive message signals transmitted from a base transmitter station. State of the art selective call receivers include microprocessors giving the receiver multifunction capabilities as well the ability to respond and display message information, including tone, voice, numeric, alphanumeric and graphics messages. This message information is transmitted using any number of coding schemes and message formats to frequency modulate a radio frequency signal.
The portable communication receiver receives and demodulates the FM radio frequency signal. A circuit known as a peak and valley measuring circuit is used to determine the maximum and minimum values of the demodulated signal. From these values a threshold may be determined and used to convert the message information in the demodulated signal to a digital signal for processing by the microprocessor. If a reference voltage representative of the desired RF carrier center frequency is available, this threshold may be employed in an Automatic Frequency Control (AFC) system to generate an estimate of the frequency error.
In prior art selective call receivers, the peak and valley measuring circuit comprises two digital-to-analog converters, one for controlling the detection of the minimum peaks (valleys) and one for controlling the detection of the maximum peaks (peaks). In applications where the symmetry between minimum and maximum peaks is important, such symmetry may be limited by the tracking error of the two digital-to-analog converters. In certain cases, any tracking error between the two digital-to-analog converters results in receiver center frequency offset.
It is desirable to provide a peak and valley measuring circuit which employs a single digital-to-analog converter to overcome the aforementioned problems.